The focus of our structural studies is the E. coli Rep helicase. The crystals are tetragonal and diffract to a resolution of 4 w. We hope to extend the resolution of diffraction so that structure determination may be achieved at moderate atomic resolution by multiple isomorphous replacement technique. This study is the first attempt to solve the structure of any helicase using X-ray crystallography. The crystal structure determination of Rep helicase will contribute significantly to our understanding of the mechanism by which DNA is unwound.